Seat belt systems of the type to which the present invention relates employ a seat belt secured at one end to an anchor which is slidably mounted on a guide rail. The anchor is connected by a drive wire to a motor or mechanical motion amplifier which operates in response to the opening and closing of the vehicle door. When the vehicle door is closed, the anchor is driven to a restraint position and holds the belt in an occupant-restraining configuration. When the door is opened, the anchor is moved to a release position and holds the belt in a releasing configuration.
It is desirable in such systems that the belt anchor be positively held at the restraint position by some sort of anchor-locking device, and the present inventor has, accordingly, invented such locking devices and disclosed them in U.S. Patent Application Ser. No. 044,196, filed May 31, 1979, and entitled "Lockable Moving Belt Anchor for Passive Vehicle Occupant Restraint Belt Systems."
When a moving anchor of either the type described in the aforementioned application of some other type arrives at the restraint location, it must stop fairly close to a specified position, particularly when the anchor arrangement includes a locking device for holding it in the restraint location. Because of normal manufacturing tolerances and changes that occur in any mechanism due to temperature, friction, wear and the like, it is difficult with known drive arrangements and control systems to stop the anchor in a precise location. Perhaps the best way to ensure proper location of the moving anchor is to include a stop and to arrange the motor or other drive device and the controls for that device in a manner that ensures that the moving anchor will be driven into engagement with the stop. This means, however, that when the anchor arrives at the stop, the drive device is abruptly stopped and is subjected to an undersirable and possibly damaging shock.
In addition to the problems that can arise due to harmful shocks on the drive system, there can also be problems with some types of lockable moving anchors involving release of the lock mechanism and movement of the moving anchor away from the restraint location. Some moving anchor-like mechanisms have gravity-actuated locking pawls that engage a locking shoulder on the moving anchor. When a vehicle having such a mechanism travels on a rough road, the pawl may shift from locking engagement, and the anchor may creep away from the restraint location and from a position in which the pawl can re-engage the shoulder.